1. Field of the Invention
The present invention relates to a field effect semiconductor device, move particularly to a gallium arsenide (GaAs) field effect transistor (FET) utilizing a two-dimensional electron gas (2DEG).
2. Description of the Related Art
A semiconductor device as mentioned above is known as, e.g., a high electron mobility transistor (HEMT) (cf. e.g., T. Mimura, S. Hiyamizu and T. Hikosaka, "Enhancement-Mode High Electron Mobility Transistors for Logic Applications", Japan. J. Apply. Phys., Vol. 20, No. 5, May; 1981, pp.L317-319). The HEMT comprises a semi-insulating GaAs substrate, an undoped (i-type) GaAs active layer, an n-type AlGaAs electron supply layer, an n-type GaAs ohmic contact layer, a source electrode, a drain electrode, and a gate electrode, e.g., as shown in FIG. 1 of the above-mentioned paper. The GaAs active layer, AlGaAs layer, and GaAs contact layer are successively grown on the GaAs substrate by a molecular beam epitaxy (MBE) process. The 2DEG accumulates at the heterojunction interface between the GaAs active layer and the AlGaAs layer, so that a 2DEG layer is formed in the top portion of the GaAs active layer. The source electrode and drain electrode are formed on the GaAs contact layer, and the gate electrode is formed on the AlGaAs layer and is located between the source and drain electrodes. In the HEMT, the 2DEG layer serves as a channel of an FET and is controlled by a voltage applied to the gate electrode.
The HEMT can be provided with an i-type AlGaAs spacer layer between the GaAs active layer and the n-type AlGaAs electron supply layer, e.g., as shown in FIG. 1 of a paper of, S. Hiyamizu, T. Mimura and structures and Their Application to High Electron Mobility Transistors", Proc. 13th Conf. on Solid State Device (1981); Japan. J. Apply. Phys., Vol. 20 (1982), Supplement 21-1, pp. 161-168. The formation of the i-type AlGaAs spacer is intended to separate ionized donor impurities from the n-type AlGaAs layer and the electrons forming the 2DEG layer. In this case, Coulomb scattering of the electrons due to the ionized impurities can be remarkably decreased, and thus a very high electron mobility of the 2DEG can be attained. Such an effect is obtained at a room temperature, but is remarkably obtained at a low temperature at which the scattering of electrons is mainly caused by the ionized donor impurities.
In order to improve the performance of a field effect semiconductor device including the HEMT, an increase of the electron concentration of a 2DEG layer is required.
The electron concentration of the 2DEG layer depends on electrons which are transferred from a donor in the n-type AlGaAs electron supply layer to the i-type GaAs active layer, by virtue of a difference between the electron affinity of GaAs and that of AlGaAs. In order to improve the electron concentration, it is preferable to increase the amount of donor impurities doped into the AlGaAs layer. The doping of donor impurities (e.g., Si) into the layer is performed by simultaneously supplying an As molecular beam, a Ga molecular beam, an Al molecular beam, and an Si molecular beam, in an MBE apparatus. In this method, however, the maximum donor concentration of Al.sub.x Ga.sub.1-x As (x=0.3) is restricted to the value of 2.times.10.sup.18 cm.sup.-3. Accordingly, the maximum electron concentration of the 2DEG layer of only about 1.times.10.sup.12 cm .sup.-2 can be attained. Such an electron concentration is insufficient for obtaining a large electric current output, so that a transconductance g.sub.m thereof is small. For example, where the semiconductor devices are used in a memory device, charging or discharging a load capacity takes a certain time, so that an operating speed of a memory device system is limited.
Recently, to improve an electron concentration corresponding to the donor concentration of the AlGaAs layer, the adoption of a superlattice technique was proposed. In this case, it is considered that, when AlGaAs is doped with Si, the donor level deepens with the result that the electron concentration of AlGaAs is not sufficiently increased due to the interaction of Al atoms and Si atoms. A superlattice multilayer structure comprising Si doped GaAs thin layers and undoped AlAs thin layers, wherein the Si atoms are kept separate from the Al atoms, is formed by an MBE process so as to correspond equivalently to the AlGaAs structure. As the result, the electron concentration of AlGaAs is increased by one order of magnitude (cf. T. Baba et al, "Elimination of Pensistent Photoconductivity and Improvement in Si Activation Coefficient by Al Spatial Separation from Ga and Si in Al-Ba-As:Si Solid System", Japan. J. Apply. Phys., Vol. 22(1983), pp.L627-L629 and NE Report "Superlattice Technique Increasing Electron Density of AlGaAs", NIKKEI ELECTRONICS, July 16, 1984, pp. 105-108).
Therefore, where the AlGaAs electron supply layer of the field effect semiconductor device is formed by using the above-mentioned superlattice technique, the electron concentration of the 2DEG layer is increased.
According to another proposed adoption of the superlattice technique, in Japanese Unexamined Patent Publication (Kokai) No. 60-28273, published on Feb. 13, 1985, an n-type AlGaAs electron supply layer of an FET utilizing 2DEG is replaced with a superlattice multilayer structure of undoped AlAs thin layers and Si doped GaAs thin layers, which AlAs layers and GaAs layers are alternately grown by an MBE process.
In these proposals for the adoption of the superlattice technique, the formation of an Si doped AlGaAs thin layer is carried out by a conventional MBE process in which an As molecular beam, a Ga molecular beam, and an Si molecular beam impinge simultaneously on a grown AlAs thin layer. However, in this conventional MBE process, the increase of a donor impurity (i.e., dopant) concentration is limited to a certain extent. Thus, in an doped GaAs thin layer of a quantum well, electrons generated from the donors and being at the quantum level are insufficient. Therefore, these proposals adopt a multiquantum well structure so as to increase the electrons for a 2DEG layer. If a doped GaAS thin layer of a quantum well is thickened (i.e., the quantum well width is increased), a quantum level of the electrons in the quantum well is decreased with result that a sheet electron concentration of a 2DEG layer is not sufficiently increased.
On the other hand, another attempt has been made to apply an Si atomic plane doping process for a doped AlGaAs layer formed on an undoped GaAs layer, in H. Lee et al, "Optimized GaAs/(Al,Ga)As modulation doped heterostructures", at Int. Symp. GaAs and Related Compounds, Biarritz, 1984, pp. 321-326. In this case, the doped AlGaAs layer is replaced with an undoped AlGaAs layer incorporating an Si atomic plane, so as to decrease the thickness of an AlGaAs layer between a gate and a 2DEG layer. The atomic plane doping technique can dope impurities to a larger amount than that of a conventional doping carried out during the MBE growth of AlGaAs. However, the above attempt does not adopt a superlattice structure.